The present invention relates to a semi-convective forced air system and method for heating glass sheets for subsequent processing. More particularly, the system and method of the present invention are used for heating low emissivity coated glass before tempering.
Forced air furnaces for heating glass sheets in preparation for subsequent processing, such as tempering, are known in the art. For example, McMaster, U.S. Pat. Nos. 4,529,380 and 4,505,671, discloses a glass sheet processing system which includes a heating furnace and a processing station for processing heated glass sheets to provide bending, tempering, bending and tempering, filming, etc. The furnace of U.S. Pat. No. 4,592,380 and 4,505,671 comprises an array of gas jets spaced above a conveyor within a heating chamber. The gas jets supply a primary gas flow directed toward the conveyor to provide forced convection heating of the glass sheets as the sheets are conveyed through the heating chamber.
The gas jets of McMaster are arranged in linear series perpendicular to the length of the conveyor and the direction of travel of the glass sheets. Each series of jets is connected to a common linear supply manifold or conduit. Each supply conduit also extends widthwise in the heating furnace, perpendicular to the length of the conveyor. McMaster teaches that the array of gas jet pumps are spaced from each other transversely to the direction of conveyance so as to uniformly heat each conveyed glass sheet over its entire width.
Heating systems such as described by McMaster appear to provide acceptable results for heating clear glass prior to tempering. Other known systems provide acceptable results for heating coated glass having an emissivity rating greater than about 0.2 prior to tempering. However, manufacturers have now begun to produce coated glass products having emissivity ratings in the range of 0.15-0.04. Prior art heating systems, including the system disclosed in U.S. Pat. Nos. 4,592,380 and 4,505,671, do not provide acceptable results for tempering glass having such low emissivity ratings. Therefore, it would be desirable to provide a system and method of tempering low xe2x80x9cexe2x80x9d glass sheeting having an emissivity rating below 0.2.
When glass sheets are conveyed into a heating furnace, the bottom surface heats at a faster rate than the top surface due to contact with the rolls of the conveyor. This causes the bottom surface to expand at a faster rate than the top surface which results in the glass bowing upward into the shape of a bowl. All of the glass sheet""s weight is supported in the center of the glass which causes the center of the glass to be overheated. This results in excessive distortion in the center of the glass which can be described as an elongated bubble. Non-uniform glass temperatures also cause the glass to oil can (or become bi-stable). Oil canning (or by-stability) and bubbling are undesirable conditions produced in the glass when the glass sheeting is not heated uniformly. Therefore, it would also be desirable to provide a method and apparatus for heating low xe2x80x9cexe2x80x9d coated glass sheets which minimizes oil-canning and bubbling.
When low xe2x80x9cexe2x80x9d glass is tempered in prior art systems, it is typically run lengthwise through the furnace due to the size of the furnace. It is also run lengthwise to mitigate the appearance of inherent distortions because the glass sheets are typically installed lengthwise down a room or hallway. However, low emissivity glass is more sensitive to heating in the longitudinal direction than it is in the widthwise direction. When glass is tempered in prior art systems, heat is only applied uniformly over the width of the glass sheet. This does not allow for separate control from edge to edge across the width of the glass. Without this control, the glass will not be heated as uniformly and the undesirable conditions of center bubble and oil canning will ensue. Therefore, it is also desirable to provide a system and method of tempering which uniformly applies heat over the entire length of the sheet in a longitudinal direction to improve aesthetic quality.
The present invention relates to a semi-convective forced air system for heating glass sheets during a heating cycle for subsequent processing such as tempering. The system and method are particularly useful for tempering low emissivity coated glass sheeting having an emissivity rating below 0.2. In accordance with the system and method of the present invention, heat is uniformly applied over the entire length of selected widthwise portions of the glass sheet to reduce or eliminate oil canning and bubbling.
The system of the present invention comprises a heating chamber, a longitudinal conveyor extending through the heating chamber, a compressed air source, a plurality of longitudinally-extending air manifolds in fluid connection with the air source, and a controller for restricting the flow of air to selected manifolds at predetermined times during the heating cycle. Each of the air manifolds is oriented parallel to the length of the longitudinal conveyor and constructed and arranged to create a downward flow of heated air toward the conveyor to convectively heat a sheet of glass on the conveyor.
The air manifolds preferably comprise elongate tubes having a longitudinal series of radially extending apertures. The apertures are oriented downwardly toward the conveyor at an angle of about plus or minus 30 degrees from vertical. The air exiting the apertures forms an angle of incidence with the conveyor of about plus or minus 60 degrees. Each of the apertures are oriented oppositely than an adjacent aperture.
The manifolds are preferably located about 4-6 inches above the conveyor. The manifolds are constructed and arranged in longitudinally-extending rows. One of the rows is preferably located above the widthwise center of the conveyor, and one of the rows is preferably located above each of the two widthwise quarter points of the conveyor. The rows, preferably the outer rows, optionally may be adjustable to different widthwise locations above the conveyor.
The conveyor preferably has horizontally-extending rolls constructed and arranged for conveying glass sheets horizontally through the heating chamber.
The air manifolds preferably comprise xc2xd inch pipe having about 0.04 inch diameter apertures longitudinal spaced about 8xc2xd inches apart. The air manifolds are constructed and arranged to simultaneously convectively heat the entire length of a selected widthwise portion of a glass sheet. A distribution manifold is arranged in fluid connection with the air source and each of the air manifolds.
A solenoid valve and flow meter are arranged in fluid connection between each air manifold and the distribution manifold. An air regulator and filter/dryer are arranged in fluid connection between the air source and the distribution manifold. A programmable computer is used to open and close the solenoids at predetermined times during the heating cycle.
The present invention also provides a method of heating glass sheet for subsequent processing such as tempering. The method comprises the initial steps of loading the glass sheet onto a longitudinally extending conveyor, and orienting the glass sheet such that the lengthwise edge of the sheet is parallel to the length of the conveyor, and conveying the glass sheet into a heating chamber. The glass sheets are then convectively heated in a specified sequence along the entire length of selected widthwise portions of the glass sheet by creating a downward flow of heated air proximate the selected widthwise portion of the glass sheet.
In a preferred embodiment, the lengthwise extending edge portions of the sheet are heated before the lengthwise central portion of the sheet. Preferably, the glass sheet is heated proximate its quarter points before the lengthwise central portion.
In a preferred embodiment, the heating step comprises constantly convectively heating only the quarter points of the sheet for the first 30-40% of the heating cycle; intermittently convectively heating only the quarter points of the sheet for the next 10-20% of the heating cycle; intermittently convectively heating only the lengthwise central portion of the sheet for the next 10-20% of the heating cycle; and, constantly convectively heating only the lengthwise central portion of the sheet for the final 20-50% of the heating cycle.
The method may include the step of transferring the glass sheet from the heating chamber to a second heating chamber of a two zone furnace.